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XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films
Yury A. Teterin,†,‡ Aleksej J. Popel,§,* Konstantin I. Maslakov,† Anton Yu. Teterin,‡ Kirill E. Ivanov,‡ Stepan N. Kalmykov,†, ‡ Ross Springell,# Thomas B. Scott,# Ian Farnan§
†Chemistry Department, Lomonosov Moscow State University, Moscow, 119991, Russia
‡NRC “Kurchatov Institute”, Moscow, 123182, Russia
§Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
#Interface Analysis Centre, School of Physics, University of Bristol, Bristol, BS8 1TL, UK
ABSTRACT: XPS determination of the oxygen coefficient kO=2+x and ionic (U4+, U5+ and U6+) composition of oxides UO2+x formed on the surfaces of differently oriented (hkl) planes of thin UO2 films on LSAT (Al10La3O51Sr14Ta7) and YSZ (yttria-stabilized zirconia) substrates was performed. The U 4f and O 1s core-electron peak intensities as well as the U 5f relative intensity before and after the 129Xe23+ and 238U31+ irradiations were employed. It was found that the presence of uranium dioxide film in air results in formation of oxide UO2+x on the surface with mean oxygen coefficients kO in the range 2.07-2.11 on LSAT and 2.17-2.23 on YSZ substrates. These oxygen coefficients depend on the substrate and weakly on the crystallographic orientation.
On the basis of the spectral parameters it was established that uranium dioxide films AP2,3 on the LSAT substrates have the smallest kO values, and from the XRD and EBSD results it follows that these samples have a regular monocrystalline structure. The XRD and EBSD results indicate that samples AP5-7 on the YSZ substrates have monocrystalline structure, however, they have the highest kO values. The observed difference in the kO values, probably, caused by the different nature of the substrates: the YSZ substrates provide 6.4% compressive strain, whereas (001) LSAT substrates result only in 0.03% tensile strain in the UO2 films.
129Xe23+ irradiation (92 MeV, 4.8 × 1015 ions/cm2) of uranium dioxide films on the LSAT substrates was shown to destroy both long range ordering and uranium close environment, which results in increase of uranium oxidation state and regrouping of oxygen ions in uranium close environment. 238U31+ (110 MeV, 5 × 1010, 5 × 1011, 5 × 1012 ions/cm2) irradiations of uranium dioxide films on the YSZ substrates were shown to form the lattice damage only with partial destruction of the long range ordering.
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